The invention relates to a method and device for the superficial heating of tissue of the type stated in the preamble of patent claims 1 and 11, respectively.
For the removal of lines and smoothing of prematurely aged skin (for example due to excessive exposure to sun), for some time laser systems such as the Er:YAG and CO2 lasers have been successfully used in dermatology as an alternative to chemical peeling. With these lasers, the skin is superficially removed. In a way similar to in the case of peeling, both the smoothing effect and the probability of undesired side-effects (scars, hyper- or hynopigmentation) increase with the depth of removal from approximately 60 xcexcm (superficial removal) to 0.8 mm (deep peeling). The removal can have the effect of partly leveling the surface of the skin, and a tightening is observed. According to current literature, the skin-tightening effect is explained by the fact that harmful, elastotic layers of skin are removed and, in the course of the subsequent healing of the wound, are replaced by a repair layer with new collagen of the type I. This effect is in principle independent of the way in which the skin was removed or irreversibly changed; it explains why mechanical dermabrasion, chemical agents or laser-induced ablations are equally effective.
The heat-induced shrinkage of collagen is discussed as a further mode of action, existing only in the case of lasers. Type I collagen fibrils shrink when heated to a temperature range between 55xc2x0 C. to 60xc2x0 C. and 70xc2x0 C. (beginning of coagulation) to a third of their length, without becoming biologically inactive. This is a process commencing immediately with the heating. Fitzpatrick et al. presume this effect to be a cause of the immediate tightening of loose and lined skin unexpectedly observed during xe2x80x9cresurfacingxe2x80x9d (R. E. Fitzpatrick et al, Pulsed Carbon Dioxide Laser Resurfacing of Photoaged Facial Skin, Arch Dermatol 132, 395-402, 1996).
Although xe2x80x9cresurfacingxe2x80x9d based on skin removal. is effective in many cases, it also has adverse side-effects. The most important are:
postoperative risk of infection (in some cases coverage of the entire face with film is required for several days)
erythema lasting for several weeks
restricted social life for at least a week
possible hyperpigmentation, less commonly hypopigmentation.
Furthermore, the newly formed skin shows deviations with respect to its structure in comparison with the remaining skin.
With the existing smoothing of lines using CO2 and Er:YAG lasers, the removal and new formation of skin are presumably the main factors determining the result. This hypothesis is made plausible by the extent of the depth of the layers of skin treated: the regions of removed skin (approximately 100 xcexcm) and irreversibly damaged, coagulated skin (CO2 laser approximately 80 to 100 xcexcm, Er:YAG laser approximately 40 xcexcm) undergo the mechanism of new skin formation. For the collagen shrinkage, on the other hand, only the region adjacent to the coagulation zone is available, lying in the temperature interval between the coagulation temperature of approximately 70xc2x0 C. and the lower limit of the collagen shrinkage temperature of approximately 55xc2x0 C. to 60xc2x0 C. For both lasers, this regionlis relatively thin in the extent of its depth during the skin removal, so that an appreciable contribution to the smoothing cannot be expected, though it is estimated to be greater in the case of CO2 lasers than, for example, in the case of Er:YAG lasers.
WO-A-97 37723 discloses a method and device of the type stated at the beginning in which the laser wavelength is chosen with respect to the thickness of the target tissue and the spectral absorption coefficients of this wavelength such that the laser radiation reaches the collagen layer in the depth of the skin without experiencing any significant absorption in the layers of skin lying above. The surface of the skin may in this case be cooled before the laser radiation is applied, in order to avoid impairment of the surface of the skin by the possible slight absorption in the upper skin. In this case, the selection of laser is restricted to certain laser wavelengths because of the required great depth of penetration into the tissue.
EP-A2-0 763 371 discloses a method and apparatus for skin rejuvenation and wrinkle smoothing in which ablation takes place by means of an Er:YAG laser, a flashlamp additionally being used to produce light pulses in the range of 600-1000 nm for heating up the collagen. The Er:YAG laser merely brings about ablation of the surface on account of the small depth of penetration of the light pulses, while the flashlamp has a greater depth of penetration and reaches the collagen. The heating of the collagen by the Er:YAG laser is obviously not regarded as adequate. For this reason, the Er:YAG laser is used in the customary way merely for removing the upper layers of skin.
The invention is based on the object of providing a method and device of the type stated at the beginning which permit improved elimination of skin damage or skin impairments, such as for example scars, lines and the like.
This object is achieved by the features specified in patent claims 1 and 11, respectively.
Advantageous refinements and developments of the invention emerge from the subclaims.
In the case of the method and device according to the invention, the heating extends from the surface of the skin, in which the absorption substantially takes place, over a relatively great layer thickness beneath the surface of the skin, it being possible in a preferred refinement of the invention to dispense completely with removal in many cases. In other cases, it may be advisable to use a combination of local removal (for example line ridges) and heating over a large region.
The range of problems so far discussed with respect to lines also apply in a similar way to the treatment of scars. In principle, renewed damage to the surface of the skin should be minimized here. A targeted heat-induced shrinkage of (scar) collagen is also possible in this case.
According to a preferred refinement of the method and device, various types of laser light sources may be used as the light source, such as for example a Ho:YAG laser, an Er:YAG laser, an Er:YSGG laser, a Tm:YAG laser, a CO2 laser or an Nd:YAG laser, these only being some examples.
In the case of all types of light sources, a considerable amount of energy can be introduced into the tissue beneath the surface of the skin by the pulse control according to the invention.
In this case, the energy level is chosen such that there is no removal, but adequate heating of the deeper-lying collagen layer is obtained by heat conduction.